1. Field of the Invention
The present invention relates to the field of nuclear magnetic resonance (NMR) apparatus, utilized particularly for medical imaging or for high resolution NMR microimaging, and has for its object a commutation device, as well as a control process using said device.
2. Description of the Related Art
At present, new high resolution NMR tests require the generation of high intensity field gradients, namely of about 100 Gauss/cm.
Moreover, the observation of spectra with a high resolution, for example less than 0.2 Hz for principal field B1 of 14.6 Tesla and a proton resonance frequency of 600 MHz, requires the generation of local field correction gradients, by means for example of a high resolution NMR spectrometer, of correction coils, also called "shim" coils.
The typical maximum value of these local field corrections is about 10 mGauss and the sensitivity of the shim in one direction, such as the Z direction, is about 10 .mu.Gauss.
As a result, the relationship between the field gradient necessary for the high resolution tests and the smallest gradient influencing the field is from 100 Gauss/1 .mu.Gauss or 10.sup.8.
Moreover, the value of the current delivered by the output amplifier of the generator supplying the coil adapted to create the field gradient of 100 Gauss/cm is about 10A.
It is therefore necessary, in order to avoid undesirable disturbances of the local field, that the generator have, at the level of its output amplifier, a stability greater than 10 A/10.sup.8 that is, 0.1 .mu.A.
Such a stability cannot be achieved at present by conventional electronic means.